A conductive polymer has a high electrical conductivity so that it have been used as a solid electrolyte for a solid electrolytic capacitor, such as tantalum solid electrolytic capacitor, aluminum solid electrolytic capacitor, and niobium solid electrolytic capacitor.
As a conductive polymer in such an application, one obtained by chemical oxidative polymerization or electrolytic oxidation polymerization of thiophene or the derivative thereof has been used.
As a dopant for performing a chemical oxidative polymerization of thiophene or the derivative thereof, an organic sulfonic [sulphonic] acid is mainly used. Among other things, it is said that an aromatic sulfonic acid is preferred. As an oxidant, a transition metal is used, and among other things, it is said that a ferric salt is preferred. Usually, a ferric salt of an aromatic sulfonic acid is used as an oxidant and dopant agent for performing a chemical oxidative polymerization of thiophene or the derivative thereof.
As a thiophene or the derivative thereof, 3,4-ethylenedioxythiophene has been often used since thereby obtained conductive polymer has a good balance of the electrical conductivity and the heat resistance so as to increase the utility of the polymer (see Patent Publications Nos. 1 and 2).
However, the engineering innovation of a solid electrolytic capacitor in using a conductive polymer as a solid electrolyte has been rapid, and further improvement in the properties is demanded, and therefore, the conductive polymer is also demanded to be improved in its properties.
Then, in order to increase the electrical conductivity, it was proposed to use 3,4-alkylenedioxy thiophene in which 3,4-ethylenedioxythiophene has been modified with a substituted alkyl group (see Patent Publication No. 3). However, in case where 3,4-alkylenedioxy thiophene is used, the heat resistance is significantly decreased, and when using it in a solid electrolyte of a solid electrolytic capacitor, the reliability of the solid electrolytic capacitor under a hot condition becomes decreased while the ESR (equivalent series resistance) becomes high (i.e., deteriorated), resulting in the lack of practicability.
Prior Art References (Patent Publications) are: Japanese Laid-open Patent Publication No. 2003-160,647 (Patent Publication No. 1); Japanese Laid-open Patent Publication No. 2004-265,927 (Patent Publication No. 2); and Japanese laid-open PCT published publication No. 2004-525,946 (Patent Publication No. 3).